a) Field of the Invention
This invention relates to a friction drive, and more particularly to a centrifugal planetary transmission in which the centrifugal force is used as the normal pressure.
b) Background of the Invention
Friction drive transmissions have a series of advantages such as structural simplicity, machining ease, quiet and even operation, improved shock-resistance and overload protection, and infinitely speed-variable drive. On the other hand, conventional friction drives incur a number of drawbacks, i.e. increased wearing, increased heat generation, lower driving efficiently, inaccurate drive ratios and larger volume. These drawbacks serve to limit the use of friction drives as they cannot be used for continuous high power transmission. The basic problem resulting in the above drawbacks is that conventional friction transmissions rely on either point contact or line contact accompanied by geometric sliding. Moreover, the large compressive force required by large friction forces wear detrimentally on the supports, such as bearings and shafts.
A friction transmission which utilizes line contact without geometric sliding, and centrifugal force as a pressing force which does not act on the supporting system has a high driving efficiency, smaller volume (high power density), accurate drive ratio and can drive continuously with large power; while at the same time maintaining the inherent advantages of conventional friction transmissions.
In order to transmit large torque or power by a small transmission volume, French patent 2599802 (abbreviated hereafter as French '802) provides a transmission comprising an input shaft, an output shaft, a planetary friction roller set which is formed by three coaxially linked cylindrical external rollers, and three internal rollers. The external rollers rotate about an axis of the external friction roller set and orbit about an axis of the internal friction rollers, and the external friction rollers contact tightly with the internal friction rollers respectively.
However, the planetary friction rollers of French '802 also contact with three external friction rollers in addition to the three internal friction rollers, so that the support of the planetary friction rollers forms a hyperstatic support system according to theoretical mechanics. The number of constrained conditions therefore exceeds the instant invention by three, caused respectively by the three additional external friction rollers; some of which bear load and some of which bear no load. The arrangement taught by French '802, thus, results in an unbalanced load distribution and increased load on the planetary bearing.
In addition, the transmission according to French '802 lacks the transmission efficiency of the present invention because if the rollers (or pulleys) are of interference fit, they cannot effectively rotate, and if the rollers (or pulleys) are of loose fit, they are not subjected to elastic deformation and cannot transmit out the torque. Moreover, the error of the radius of curvature in the case of external friction contact is increased, thus resulting in insufficient contact strength.